Known light modulation devices include those utilizing an electro-optical effect, a magneto-optical effect, an acoustic optical effect, etc.
Light modulation devices utilizing an electro-optical effect use a change in refractive index on electrical field application, and those utilizing a magneto-optical effect use rotation of the plane of polarization on magnetic field application. Those utilizing an acoustic optical effect use a phenomenon that, for example, when a radiofrequency voltage is applied to a transducer composed of comb type electrodes formed on a waveguide to produce a surface acoustic wave according to the frequency of the input signals, the angle of polarization of the guided light changes by the interaction between the guided light and the surface acoustic wave.
Since the conventional light modulation devices thus achieve light modulation by application of an electrical or magnetic field, there is a limit in possibility of increasing precision and speed. Hence, devices enabling light modulation by using light are expected to be very useful.
Light modulation devices using light which have been proposed to date include (1) devices comprising a combination of a photoconductive element and a liquid crystal element, (2) devices using a mixture of a ferroelectric liquid crystal and a photochromic compound (see The Abstract of the 17th Liquid Crystal Lecture Meeting, p. 246), and (3) devices in which a photochromic compound is orientated to have a transition moment in a given direction and a change in birefringence of the photochromic compound accompanied by photoisomerization is utilized (JP-A-2-190827, the term "JP-A" as used herein means an "unexamined published Japanese patent application").
With reference to use of a light modulation device as an optical memory, it has hitherto been proposed to use a photochromic compound to provide an erasable optical memory. However, reading of the memory according to the conventional technique requires exposure to light of the wavelength region in which the photochromic compound used shows absorption. Since the light for reading also serves as light for erasion of the memory, there has been involved a great problem for practical use that exposure to such light results in destruction of the memory.
To avoid this, it has been suggested to use light of the longer wavelength region in which a photochromic compound used shows no absorption to induce a change of the photochromic compound other than a change in absorbance, such as a change in optical rotation (see JP-A-1-246538) or refractive index anisotropy (see The 58th Spring Anniversary of Japan Chemical Society, The Abstract of Lecture Meeting (1989), 31H30).
Attempts have also been made to use a mixture of a photochromic compound and a liquid crystal material to bring about a greater change. For example, a method in which a liquid crystal material is mixed with a chiral photochromic compound, and a cholesteric liquid crystal phase is changed by photoisomerization of the photochromic compound (see The 52th Spring Anniversary of Japan Chemical Society, The Abstract of Lecture Meeting (1986)) and a method in which a liquid crystal material is mixed with a compound which undergoes a great structural change through a photochromic reaction, and a change in circular dichroism spectrum is caused by photoisomerization have been proposed.
Further, for the purpose of preventing deterioration with time caused by fluidity of a liquid crystal, a method of using a high polymeric liquid crystal and a method in which an orientation state of a liquid crystal is changed by photoisomerization of a photochromic compound immobilized on a substrate (see JP-A-1-251344).
The above-described light modulation devices (1) and (2) essentially utilize an electro-optical effect and give rise to a problem of electrical power consumption. In the case of the devices (1), a complicated step is required for laminating a photoconductive element and a liquid crystal element. In the case of the devices (2), although the structure is simple, the modulation mode achieved is only binary because the bistability of a ferroelectric liquid crystal is used, and the working temperature range is narrow. In the case of the devices (3), the change in birefringence accompanied by photoisomerization is too small for obtaining a sufficient contrast.
Where the light modulation devices so far proposed are applied to optical memories, none of them is satisfactory due to various disadvantages involved on practical use. For example, the methods of reading any change other than a change in absorption of a photochromic compound are hardly put to practical use because the change in optical properties attained is small. The methods of using a photochromic compound in combination with a liquid crystal material have a disadvantage in that the liquid crystal flows with time to make the memory unclear and also the memory has poor thermal stability and poor durability on repeated use. The system utilizing a phase transition of a high polymeric liquid crystal encounters difficulty in completely erasing the memory. Thus, the conventional memory materials using a photochromic compound have various problems and have not yet been put to practical use.